1. Field of the Invention
The present invention relates to a method of storage for a storage device used in telecommunication system, in particular telephone exchange systems, in which at least one overall memory is provided which is constructed for formatting from a plurality of storage elements, e.g. storage sectors, of standard storage volume, e.g. a magnetic disk for magnetic disk memories. By way of a selectable number of storage elements, storage sections of specified size are formed from storage elements which are related in terms of their drive and which can be successively individually driven by way of addresses. Reserve storage elements contained in a reserve section of the overall memory are protected from, i.e. excluded from a direct access which serves to form the storage sections which are related in terms of drive. In the event of a defect in a storage element, the same is assigned a reserve storage element in that the selection addresses of the reserve storage element is stored for the defective storage element, as a result of which, during the individual successive drive of the storage elements of a storage section containing a defective storage element, when the latter is reached, instead of itself, the reserve storage element assigned thereto is included into the drive cycle, and is subjected to a write, read, or erasure process in place of the defective storage element.
2. Description of the Prior Art
In known memories which operate in accordance with this method, in addition to the storage element main component which serves for the primary purpose of storing any items of information, each of the storage elements additionally contains an address array in which its own address is entered. The drive can take place by way of a search procedure using a predetermined address which is compared, on each occasion, with the address read out from the address array of a storage element; in the event of identity, the storage element which is to be driven is then operated in each search process. However, the drive can also take place by means of items of selection data which, for example, indicate the row and column in a coordinate array or cylinder and sector on a magnetic disk. In this case, the address stored in the address array in question serves to monitor a previously-corrected performed drive process.
It continually occurs that the storage medium from which the storage elements for the overall memory, including the storage element main component and address array, is faulty at some locations. As a rule, this applies only to the individual storage elements. A fault of this kind can consist in that a storage element can no longer be recorded upon, read or erased. Therefore, it has already been provided that, in addition to the aforementioned storage sections in the overall memory, a reserve section of reserve storage elements must be provided. These cannot be directly used to form storage sections in the same way as the storage elements. From the latter it is possible to form storage sections consisting of arbitrary members of storage elements. A storage section is formed for a specific storage purpose, e.g. for storing a program or a subsidiary program or data relating to a switching network configuration or the like. Since the storage volume requirement generally differs from storage purpose-to-storage purpose, it is possible to select the number of storage elements in the formation of a storage section. A memory access device has free access to the storage elements from which, depending upon the storage volume requirement, it conforms storage section of arbitrary size regarding the number of storage elements which are related in terms of address. Here, an exclusion consists of the reserve storage elements of the reserve section. Therefore, the memory access device is denied free access to the reserve storage elements.
Rather, in the event of a defect in a storage element, the reserve storage element can be individually assigned thereto, and, in fact, as a substitute for the storage capacity which has become lost as a result of the defective storage element. In this assignment procedure, one reserve storage element is always individually assigned to a defective storage element. For this purpose, it is provided that a pointer character be stored in the address array of a storage element when the storage element in question is defective, and that it should be assigned a reserve storage element in that the address of the defective storage element is entered into the address array. If the storage elements are successively driven for the execution of a write, read or erasure process, and if in so doing the defective storage element is reached, during the reading of its address array the pointer character simultaneously appears, therefore indicating the existence of the defect and resulting in the fact that, instead of this storage element, the reserve storage assigned thereto is driven. This is carried out by driving the reserve section, and by reading the address arrays of the reserve storage elements in succession. In the event of identity between the address which has currently been read with the address of the defective storage element in question, this search procedure is terminated, since the reserve storage element assigned thereto has been found, and therefore has been driven by this search procedure.
The previously-described system of driving a reserve storage element assigned to a defective storage element is a time-consuming procedure. Furthermore, it is possible that not only the main component of the storage element, but also its address array may be faulty. In this case, the address array cannot be read or the fault will manifest in the result of the reading. Then, the described search is performed in the reserve storage section. The pointer character may then be absent, or can be falsely simulated. The one or the other of these two faults can result in the fact that the defective storage element, which has already been assigned a reserve storage section, is wrongly reassigned a reserve storage element, or that the reserve storage element assigned to a defective storage element is searched for when as assignment required for this purpose has not taken place. In the final analysis, all such misfunctions, i.e. a double assignment of reserve storage elements to a defective storage element or the absence of such assignment, are, in fact, detectable. However, all the additional procedures required for purposes of rectification involve additional expense for corresponding programs and, in particular, cost time, therefore slowing down the procedures flows in the drive of the storage elements of a storage section.
In the event of defects of this kind, firstly read repetitions are carried out, which as is known can be extremely time consuming. Here, the same storage element is redriven. Such read repetitions must be performed not only on the first occasion of a fault in the address array, but also when the effective storage element (defective in the address array) has already been assigned a reserve storage element at an earlier time, this must be repeated on each occasion. If such read repetitions do not lead to a clearly-defined, utilizable read result, in the reserve section a search for a reserve storage element already assigned to the respective storage element is begun, since it is possible that the defect which has occurred has already been recognized on the occasion of an early case operation and has lead to a corresponding assignment procedure. If, however, no reserve storage element bearing the address of the defective storage element is found, this implies that an assignment procedure of this type has not yet taken place and must be performed at the time in question. These procedures are all extremely complicated and time consuming.
It is also known that faults of a sporadic type additionally occur, i.e. faults which occasionally occur at random do not repeat. Such defects can be produced, for example, as a result of traveling particles of dust. If, during operation, a sporadic fault of this type has lead to the assignment of a reserve storage element to a defective storage element and a corresponding item of information has been stored in the reserve storage element, and if this defect has disappeared at random on the next occasion of operation, the item of information in question is read not only from the reserve storage element but from the original storage element to which the reserve storage element has already been assigned on the earlier occasion of operation, i.e. during the earlier processing. This can lead to the fact that a false item of information is read.
Furthermore, a defect can consist in that a storage element initial character, which marks the beginning of a storage element, has become illegible. In the case of a defect of this kind, the search for this storage element, initial character can be continued and simulated by a later different character which can be interchanged with the storage element, initial character as a result of which the entire storage process takes place with a delay relative to the address array in question and consequently leads at random to false results, or else an interchangeable storage element initial character of this kind is not discovered and a time monitoring process signals that the search procedure has not lead to a result. Defects of this kind possess unpredictable and virtually uncontrollable consequences. Therefore, it has already been proposed for a memory arrangement operating in accordance with the method described above, that in an additionally provided special memory, the addresses of all the defective storage elements be collected and stored in individual assignment to the addresses of the assigned reserve storage elements. Furthermore, during a write, read or erasure process of a storage element, on the basis of the selection address of the next storage element to be driven, which address is obtained from the selection address of the storage element in question as a result of the address succession, by way of a search procedure in the special memory it should be checked whether the selection address of this storage element is stored as defective. Furthermore, and in the case of a positive result, the selection address of the reserve storage element in question, which is stored in association therewith, should be read, and the reserve storage element should be driven and should perform the write, read or erasure process. If, under these conditions, a storage arrangement having relatively rapid access is used as a special memory, on each occasion during the processing (read, write or erasure) or a storage element, the selection address of the next storage element to be driven can be obtained on the basis of the address section, whereupon the special memory can be searched to establish whether this selection address is stored therein. If so, the drive address, stored together with this selection, i.e. the drive address of the defective storage element, of the reserve storage element assigned to the defective storage element, is read and is used for the next drive operation of the current processing in place of the selection address of the defective storage element. If this is not so, which however is not ascertained until the entire contents of the special memory has been searched, the drive takes place with the drive address of the storage element in question, since it has not been assigned a reserve storage element because of the fact that it is not defective.
In comparison to the storage method described in the introduction, the above-described proposed more novel method of storage results in a substantial gain in time. However, this is heavily dependent upon how fast the read processes can be handled in the special memory. This, in turn, is dependent not only upon the operating speed of the special memory, but also upon its level of fullness, i.e. a varying and non-predictable influencing parameter. If the length of the read procedures in the special memory exceeds the duration of the processing (read, write and erase) of a storage element, time losses occur. The processing procedure of the next storage element cannot begin properly on each occasion. In magnetic disk memories or magnetic drum memories in which it is well known that the storage medium constantly circulates, this can have a negative outcome inasmuch as in such cases it is necessary to wait for a complete rotation of the disk or drum, which substantially increases the time losses.